1. Technical Field
Embodiments of the present invention relate to methods and apparatus for enhancing reception of signals more, and particularly, to techniques for using post combiner radio synchronization (SYNC) detection to improve multipath performance of a Rake receiver.
2. Discussion of the Related Art
When telecommunication or radio signals are transmitted from one radio to another, the radio signal may “bounce” of various objects such as buildings or terrain. These bounced signals arrive at the destination receiver at differing times and are referred to as multipath signals. To assist in recovering the multipath signals, some radio applications will use a “Rake” receiver. A Rake receiver is a type of receiver that employs a plurality of sub-receivers or correlators referred to as “fingers” that are each assigned to a different multipath. In this regard, the fingers are like the tines of an ordinary garden rake with each finger collecting signal energy much in the same way that the tines of the garden rake collect leaves. Each finger is designed to detect a single multipath component that can be combined with other multipath components to improve reception.
In a conventional radio Rake receiver, multipath detection (i.e., the Rake finger tap selection) is conditioned upon detecting a radio synchronization or “SYNC” event. That is, conventionally, correlation peaks from a correlation peak detector are first compared with a SYNC threshold value. The SYNC is declared as detected if the correlation peak is greater or equal to the threshold. Once the SYNC is detected, the correlation peaks are then passed on to the Rake finger tap selector along with the time tag information indicating tap locations for multipath detection. The SYNC detection point determines the start time for the Rake finger selection and combiner processing. However, this SYNC triggered multipath detection will be good in a multi path environment only when the first significant path signal is greater than the SYNC threshold and received ahead of the other path signals.
In many situations, especially in a Rayleigh fading environment, it is likely that the first significant path signal may not cross the SYNC threshold. In these cases, conventional detection methods will produce incorrect multipath detection and select incorrect (fewer) Rake fingers for signal combining. In other words, SYNC is declared late in the multi-path window so that most of the possible Rake fingers are omitted. As a result, the Rake receiver's performance will degrade significantly.